Ah, vibrato...such a complex topic, since it cannot be seen but only heard. In general, if the ear is drawn to vibrato initially, it is probably too wide or too slow...a beautiful tone is shaded with vibrato, not dominated by it. In general, there should be 4-6 pulses of vibrato per quarter note pulse. Experiment with different speeds, especially on diminuendo exercises. Take the vibrato out as you go softer: next time, keep the vibrato spinning smaller and faster as you get softer. PLAY! Make your long tone exercises a laboratory for success by trying all kinds of speeds/widths of vibrato in each register and in different dynamics. Remember to keep the vibrato on the low side of the pitch so that you are not going sharper as a general trend when using vibrato.